Why would she be covering up? We're going to talk about that. She's gorgeous. We're going to start with the trial elizabeth was talking about. A utah doctor who is accused of killing his wife so he could be with his mistress. Aditi roy has all of the details on the case that took years to make. Reporter: Today, martin McNEIL IS BEGINNING THE TRIAL OF His life. The former physician and mormon sunday schoolteacher is ready to prove his innocence to the newly-selected jury. He says he did not kill his wife so he can carry on an affair with another woman. While he is very stressed, he also is quite relieved to finally have the opportunity for the trial to occur. Reporter: Michelle macneill was discovered dead in 2007. The medical examiner found drugs in her system. Prosecutors and his own DAUGHTERS say MacNeill gave his wife a deadly dose of prescription painkillers while she was recovering from a face-lift, one they say he insisted she had. He had a plan to murder my mother. And this was just part of his plan. Reporter: This morning, more than six years after their mother's death, eight jurors will finally be seated to decide their father's fate. Potential jurors answered 22 pages of questions, including, would the fact the defendant was having an affair at the time of his wife's death cause you to believe you would not be a fair juror? Are you inclined to believe the testimony of a child witness between the ages of 8 and 15, more so or less so than an adult witness? Explain. 12-YEAR-OLD aidan MacNeill, the youngest daughter and the one that discovered michelle's body is expected to testify. Alexis and rachel, who held up pictures of their mother when their father was in court yesterday. Both sides poised for opening arguments today. This isn't a popularity contest. We need to let the jurors know that. This is much more serious than that. Much more important. Reporter: Martin macneill, facing first-degree murder and a fight for freedom. For "good morning america," aditi roy, abc news, provo, utah.

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